At the time of filing this application, there are two very large scale, commercial plants extracting bitumen from oil sand in the Fort McMurray region of Alberta, Canada.
Oil sand comprises sand grains that are water wetted or individually coated with a thin sheath of connate water. The oil or bitumen is present as a continuous matrix in which the wet grains are embedded. In addition, clay and silt particles (termed "fines") are contained in the oil sands. These fines will pass through a 44 micron screen. Of interest in connection with the present invention, the fines include particles having a size less than about 300 nanometers (termed "ultrafines").
The bitumen is extracted from the oil sand using the hot water extraction process. This process is described in the prior art literature and, in the case of the plant owned by the present assignees, involves:
Conditioning the oil sand by mixing it with hot recycled process water and a small amount of process aid (commonly NaOH) in a rotating horizontal drum (termed a `tumbler`). Steam is sparged into the produced slurry to ensure that the exit temperature is about 80.degree. C. In the course of conditioning, the viscous bitumen is heated and is separated from the sand grains; it is released into the water phase in the form of minute flecks. At the same time, small air bubbles are entrained in the slurry. Fine bitumen flecks coalesce and form larger globules that contact and coat air bubbles, thereby becoming buoyant; PA1 The product slurry leaves the tumbler and is screened, to remove oversize material; PA1 The screened slurry is then "flooded" or diluted with additional hot water to produce a slurry containing about 50% solids by mass, based on the original oil sand feed; PA1 The diluted slurry is then retained in a large thickener-like vessel (called a "PSV") for about 45 minutes. In this vessel, the aerated bitumen rises and is recovered as an overflow `primary` froth product. Unaerated bitumen may be recovered in this vessel by coalescence with aerated bitumen or with the already recovered oil at the froth/water interface. The sand settles and leaves the vessel as an underflow stream containing water and some bitumen. In the mid-section of the PSV there exists a watery mixture comprising relatively non-buoyant bitumen and fines--this mixture is referred to as `middlings`; PA1 A stream of PSV middlings is mixed with PSV underflow and the mixture is introduced into a cone settler referred to as the tailings oil recovery vessel ("TORV"). In the TORV, the feed mixture is deflected radially as it is fed in and is spread outwardly and horizontally. The out-moving mixture is contacted from below by an upwelling stream of aerated middlings. A secondary yield of froth is produced. The underflow from the TORV, comprising solids, water and some bitumen is discharged as tailings; PA1 A stream of middlings is withdrawn from the TORV and is fed to a bank of sub-aerated flotation cells. Here the middlings are subjected to relatively intense aeration and mixing. Bitumen contained in the middlings is recovered in the form of a `secondary` froth that is heavily contaminated with water and solids. The tailings underflow stream from the flotation cells joins the TORV tailings and the product is discharged to a tailings pond; and finally PA1 tailings is discharged to a pond and a clarified upper layer of water is recycled back to extraction as a majority of the process water. PA1 to achieve high total bitumen recovery, in the order of at least 90%, PA1 to maximize recovery of bitumen in the form of primary froth, as opposed to secondary froth, and PA1 to maintain a low bitumen loss associated with the tailings, typically below about 3%. PA1 The pond presently covers about 15 km.sup.2 and has a present depth up to about 45 m in the deepest portions; PA1 The sand, fines and some bitumen settle to leave a clarified top layer of water having a depth of about 5 m. This layer contains about 0.1 wt. % to 1.0 wt. % suspended solids; PA1 The clarified water is recycled as process water after a pond retention time of about 9 months; PA1 A "sludge" layer, of only slightly increasing density and containing only a small content of coarse solids, extends downwardly from the clarified water layer. The sludge layer presently has an average thickness of about 22 m. Its maximum solids content reaches about 30 wt. %. Typically, the sludge comprises 87% water (by volume), 1-2% bitumen and 11-12% solids. The sludge contains a high proportion of ultrafines; PA1 At a solids concentration of about 30 wt. %, the sludge is able to trap and suspend descending coarse solids which would otherwise settle out at the bottom. Thus the bottom layer of the pond is a sludge of increasing density and coarse solids content. At the base of this layer, the solids content may reach 60 wt. %. PA1 instead of forming a gradual consistent gradient in the distribution of the particles in the column, the sludge would separate to form an upper clarified portion and a lower solids-containing portion, with a clear demarcation between them; PA1 when the cylinder, containing the settled charge, was tapped, the solids-containing portion appeared to move as a unit, indicating some degree of cohesiveness; and PA1 if a liter of settled sludge in a cylinder was diluted with a liter of clean water and the charge was then shaken and again allowed to settle, the particles would congregate in the same volume that the solids-containing portion had occupied prior to dilution--the added water would join the clarified water portion. PA1 that the ultrafines take the form of platelets having a radius to thickness ratio in the order of about 30:1 and having a thickness in the order of 1 to 10 nanometers; and PA1 that the ultrafines are attracted to each other and form a collection of interconnected aggregates comprised of flocs, thereby producing a thixotropic gel. PA1 treating the process water delivered to the pond by ensuing sufficient dispersant enters the process water circuit to disperse ultrafines and hinder formation of flocs; PA1 delivering the treated process water to a beach at the pond's inlet where some ultrafines are removed from the recycled process water; PA1 withdrawing the process water and residual ultrafines from the pond's outlet for maintaining the volume of the process water in fie pond sufficiently small so that the retention time of the process water and residual ultrafines is short enough that the ultrafine particles remain in a dispersed state and are prevented from forming flocs; and PA1 repeatedly withdrawing the process water and residual ultrafines from the pond's outlet and recycling them to the plant so that the ultrafines content increases to a steady state concentration.
In general then, the hot water extraction process involves mixing large quantities of recycled process water with oil sand in preparation for a first `spontaneous flotation` step (in the PSV) to yield a relatively high quality froth and then one or more subsequent `induced flotation` steps (in the TORV and sub-aerated cells) to yield `secondary` froths that contain relatively higher concentrations of water and solids. An important factor in the process is therefore to maximize the proportion of the bitumen in the oil sand that is recovered in the form of primary froth. It is therefore obviously important to minimize the loss of bitumen with the tailings (comprising water, solids, and unrecovered bitumen) so as to maximize the total amount of bitumen recovered. A portion of the solids in this tailings stream forms a sludge.
As previously stated, the present invention is concerned with reducing the formation of sludge. However, any effort to reduce sludge formation needs to be compatible with seeking:
As a general rule, the presence of fines in the recycled process water has been considered to be detrimental. A high fines content will hinder the formation of froth and oil recoveries will be reduced.
As previously stated, the tailings are discharged into a pond, which is enclosed by constructed dykes. More particularly, the tailings from the plant are discharged onto a sloping `beach` at the edge of the pond. As the tailings fan out across the beach, much of the sand quickly settles out and joins the beach. In the course of this, the contained tailings water and fines associated therewith are also trapped by the sand forming the beach, in a ratio of about 1 weight unit of water per 4 weight unit of mineral.
The balance of the tailings joins the pond contents. The tailings are fed in at one side of the pond and clarified water is recycled to the plant from the other side of the pond, for use as extraction process water.
At applicants' commercial plant:
The volume of sludge generated is very large. Applicants' commercial plant processes about 350,000 tonnes of oil sand per day. About 0.2 m.sup.3 of sludge are produced for each tonne of oil sand processed. This sludge layer is persistent and stable, requiring an ever-increasing pond capacity. The cost of providing the dykes to contain the increasing volume of sludge is very high. Therefore it is highly desirable to densify or eliminate the sludge with the objective of reducing the volume of pond space needed and reducing the dyke cost.
Solutions to the problem have been offered in the prior art patent literature.
In general, the prior art teaches the view that the fine solids do not settle and form a dense bed because they are mutually repelling and therefore will not densify in water beyond a density in the order of 30 wt. %. The prior art solutions therefore commonly take the form of adding a flocculant to the tailings, to cause the fines to agglomerate and densify.